304 PHYSIOLOGY, SYMBIOSIS, ETC. 



spores. So far as the algae of tropical soils are concerned the dry 

 spores oi A'ostoc sp. and O. brevis can tolerate 2 min. at 100° C, the 

 wet spores 20 min. at 60-70° C, and the vegetative filaments 

 10 min. at 40° C, this latter being a temperature that is fre- 

 quently reached on open ground in such regions. Acidity and 

 alkalinity do not appear to be of any great importance, although 

 members of the Chlorophyceae usually thrive better on basic soils. 

 It has been demonstrated that Anabaena and Nostoc can fix 

 nitrogen from the air in the presence of light, but other soil algae 

 apparently do not possess this power unless they occur in combina- 

 tion with bacteria, and even then the actual fixation is probably 

 carried out by the bacteria. It has been found by De (1939) that 

 Anabaena will only fix nitrogen from the air so long as nitrate is 

 absent from the soil. The combination of bacteria and algae fix 

 nitrogen better than the bacteria do alone, so that the algae must 

 act as a kind of catalytic agent, and it has been suggested that they 

 {a) provide carbohydrate, and hence energy, for the bacteria, or 

 {b) remove the waste nitrogen compounds, since it has been shown 

 that if these accumulate bacterial activity is reduced. In some cases 

 the algae play a part in aeration because of the oxygen they produce 

 during photosynthesis, and in this connexion it may be mentioned 

 that unless certain species are present in the soil of rice fields during 

 the period they are waterlogged the aeration deteriorates and the 

 rice becomes much more susceptible to disease. Rice is also capable 

 of growing in the same field year after year without being manured, 

 and it has been demonstrated that this is due to the fixation of 

 nitrogen by the algae present in the soil. 



REFERENCES 



Physiology. BiEi ., R. (1938). Jb. Wiss. Bot. 86, 350. 



Soil Algae. De, P. K. (1939). Proc. Roy. Soc. Ser. B, 127, 121. 



Physiology, du Buy, H. G. and Olson, R. A. (1937). Amer.J. Bot. 24, 



609. 

 Physiology. Ehrke, G. (193 i). Planta, 13, 221. 

 Soil Algae. Fritsch, F. E. (1936). Essays in Geobotany in honor of 



W. A. Setchelly p. 195. Univ. California Press. 

 Soil Algae. Fritsch, F. E. and Haines, F. M. (1922, 1923). Ann. Bot. 



36, i; 37, 683. 

 Physiology. Haas, P. and Hill, T. G. (1933). Ann. Bot., Lond., 47, 55. 

 Physiology. Hanson, E. K. (1909). New Phytol. 8, 337. 

 Physiology. Hyde, M. B. (1938). J. Ecol. 26, 118. 



